The present invention relates generally to an olefin resin composition for powder molding, and more particularly to a resin composition for powder molding that is improved in terms of powder flowability and moldability.
Skin materials for automotive interior parts such as instrument panels, console boxes, door trims and glove boxes have so far been substantially molded of vinyl chloride-based resin materials. In consideration of recent environmental problems, however, there is growing demand for parts molded of easy-to-recycle olefin resin materials classified as non-halogen type resins.
Numerous olefin resin compositions have thus been put forward as powder-molding materials (for instance, Japanese Patent Application Laid-open (A) Nos. 7-178742, 6-226763, 8-217927, 6-170871, 5-1183 and 5-5050). However, these compositions are so poor in powder flowabillty that particles are susceptible to agglomeration, often resulting in products having thickness variations, deficiencies, and pinholes.
To solve such poor-flowability problems with these powder-molding olefin resin composition, the deposition of inorganic dusting agents such as finely divided talc, calcium carbonate, calcium silicate and aerosol onto the surfaces of resin particles has generally been relied upon. However, the presence of the inorganic dusting agent on the surfaces of resin particles gives rise to an increase in the resin surface""s melting viscosity, which in turn results in the need of carrying out molding at considerably high temperatures for considerably long periods of time. This eventually makes moldability worse.
JP-A 6-106553 comes up with a process wherein finely divided resin powders having an average particle diameter of up to 30 xcexcm such as those based on polypropylene resins, polyethylene resins and vinyl resins are used as dusting agents for thermoplastic elastomers comprising ethylene xcex1-olefin copolymer rubber and olefin resins. However, since these components have a glass transition temperature that is lower than room temperature, the temperature of the resin composition rises locally upon repeated powder molding cycles and so the particles are likely to agglomerate. As a consequence, the flowability of powders drops, often resulting in sheet moldings having thickness variations, and pinholes. After long-term storage, the flowability of such resin compositions becomes extremely worse, ending up with a drop of powder moldability.
Situations being like this, an object of the present invention is to provide an olefin resin composition that ensures improved powder flowability and long-term storability, and has moldability so improved that even upon powder sintering and molding, there is no need of elevating molding temperature and extending molding time or there is no moldability-disturbing factor.
The inventors have now found that the aforesaid object is achieved by the incorporation into an olefin resin of a non-halogen type thermoplastic resin having a specific glass transition temperature and a specific particle diameter and shape. On the basis of such findings, the present invention has now been accomplished.
More specifically, the present invention provides an olefin resin composition for powder molding, comprising (A) 100 parts by weight of an olefin resin having a glass transition temperature of up to 25xc2x0 C. and (B) 0.5 to 30 parts by weight of a non-halogen type thermoplastic resin having a glass transition temperature in the range of 60 to 200xc2x0 C. an average primary particle diameter in the range of 0.1 to 10 xcexcm and a sphericity in the range of 0.8 to 1.0.
The olefin resin composition for powder molding according to the present invention is excellent in powder flowability and long-term storability, and is improved in terms of powder moldability as well.
The present invention is now explained at great length.
The olefin resin (A) used herein includes a homopolymer or copolymer of olefin monomers having 2 to 10 carbon atoms such as ethylene, propylene and 1-butene, and a copolymer of at least 50% by weight of one or two or more such monomers and other monomers copolymerizable therewith. The olefin resin (A) should have a glass transition temperature (Tg) of up to 25xc2x0 C., and preferably up to 20xc2x0 C. When the Tg of component (A) is too high, high temperature is needed to mold powders. In addition, the resulting molded product has limited use, because it is hard, and cannot be released out of the mold unless its configuration is simple. It is noted that Tg may be determined by means of a differential calorimeter.
Typical examples of such an olefin resin (A) are ethylene resin and propylene resins.
The ethylene resins include high density polyethylene, medium density polyethylene, low density polyethylene and linear low density polyethylene as well as ethylene copolymers containing at least 50% by weight of ethylene such as ethylene-propylene copolymers, ethylene-propylene-diene copolymers, ethylene-1-butene copolymers, ethylene-1-hexene copolymers, ethylene-1-heptene copolymers, ethylene-1-octene copolymers (EOR), ethylene-4-methyl-1-pentene copolymers, ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, ethylene-acrylic acid alkyl ester copolymers, ethylene-methacrylic acid copolymers and ethylene-methacrylic acid alkyl ester copolymers. The preferred ethylene resins are linear low density polyethylene, ethylene-1-butene copolymers, ethylene-1-hexene copolymers, ethylene-1-heptene copolymers and ethylene-1-octene copolymers (EOR).
Exemplary propylene resins are propylene homo-polymers and copolymers of at least 50% by weight of propylene and other xcex1-olefins. The xcex1-olefins used herein, for instance, are ethylene, 1-butene, 4-methyl-1-pentene and 1-octene. It is noted that the copolymers of propylene and xcex1-olefins include random copolymers, alternating copolymers and block copolymers, and may be produced by any desired production processes.
According to the present invention, the olefin resin (A) also includes an aromatic thermoplastic elastomer exemplified by styrene-butadiene-styrene block copolymers (SBS) and styrene-isoprene-styrene block copolymers (SIS) as well as their hydrogenated products (SEBS, SEPS).
According to the present invention, the aforesaid olefin resins (A) may be used alone or in combination of two or more.
Referring to the properties of the olefin resins (A) used herein, they should have a melt flow rate of at least 5 g/10 min., and preferably at least 20 g/10 min. (MFR: as measured according to JIS K 7210 and at 230xc2x0 C. under 2.16 Kg load for resins having a melting point of 160xc2x0 C. or higher and at 190xc2x0 C. under 2.16 Kg load for resins having a melting point of less than 160xc2x0 C.). With resins having an excessively low MFR, viscosity sintering becomes difficult and the resulting molded product is likely to have defects such as pinholes.
The non-halogen type thermoplastic resin (B) used herein is a polymer in which any halogen type compound is not used as a monomer, and has a glass transition temperature (Tg) in the range of 60 to 200xc2x0 C., and preferably in the range of 80 to 130xc2x0 C. At an excessively low Tg, the flowability of powders tends to become worse in the process wherein molding temperature rises. At an excessively high Tg, to the contrary, the melting properties of powders may possibly degrade upon molding.
The non-halogen type thermoplastic resin (B) has an average primary particle diameter in the range of 0.1 to 10 xcexcm, and preferably 0.5 to 5 xcexcm. When the average primary particle diameter is excessively small, the powder flowability of the present composition tends to dwindle with time in the case where the amount of the thermoplastic resin (B) added is relatively small. Where the thermoplastic resin (B) added is large, the resulting molded product is likely to have defects such as color variations. On the other hand, when the average primary particle diameter is excessively large, the thermoplastic resin (B) has a risk of hardly functioning as a dusting agent. The average primary particle diameter may be determined by dispersing powders of the non-halogen type thermoplastic resin (B) in water, shaking the dispersion for 1 minute in an ultrasonic shaker operating at an oscillation frequency of 50 kHz, and permitting the shaker to stand for 3 minutes to find a integral particle diameter distribution by means of a centrifugal sedimentation turbidimetry method using the resulting suspension. The average primary particle diameter is then given by a particle diameter with a cumulative value of 50%.
The sphericity of the non-halogen type thermoplastic resin (B) should be in the range of 0.8 to 1.0. When the non-halogen type thermoplastic resin having too low sphericity is used as the dusting agent, any sufficient improvement in power flowability is not obtainable. Referring to how to determine the sphericity, a sample is first observed at 10,000xc3x97magnifications under a transmission electron microscope (TEM) to check whether polygonal or acute particles are found or not. If they are not found, then a photograph is taken of the sample. From the photograph the mean breadth-to-length ratio is found for 100 particles.
Vinyl resins are preferable for the non-halogen type thermoplastic resin (B). Exemplary vinyl resins are acrylic resins that are polymers of ethyl arcylate, methyl methacrylate or the like; aromatic vinyl resins that are polymers of styrene, xcex1-methylstyrene, vinyltoluene or the like; vinyl cyanide resins that are polymers of acrylonitrile, vinylidene cyanide or the like; vinyl ester resins that are polymers of vinyl acetate, vinyl propionate or the like; and vinyl ether resins that are polymers of methyl vinyl other, hydroxybutyl vinyl ether or the like.
Merits of acrylic resins containing as monomers acrylates or methacrylates having various alkyl groups are that such monomers are available with relative ease and Tg can be easily varied depending on the length of alkyl groups. Typical acrylic resins are homopolymers of methyl methacrylate (Tg: 105xc2x0 C.), ethyl methacrylate (Tg: 65xc2x0 C.), isopropyl methacrylate (Tg: 81xc2x0 C.), t-butyl methacrylate (Tg: 107xc2x0 C.) and phenyl methacrylate (110xc2x0 C.). It is here noted that the referred-to values of Tg are measured on a homopolymer basis. In the present invention, these resins may be used immediately as the dusting agent.
Monomers that give homopolymers having a Tg value of lower than 60xc2x0 C., for instance, n-butyl methacrylate (21xc2x0 C.), n-octyl methacrylate (xe2x88x9220xc2x0 C.) and n-hexyl methacrylate (xe2x88x925xc2x0 C.) should preferably be copolymerized with other monomers that yield homopolymers having high Tg values, for instance, the aforesaid acrylic or methacrylic ester as well as styrene (105xc2x0 C.) and xcex1-methylstyrene (101 to 125xc2x0 C.), so that Tg values of 60xc2x0 C. or higher can be obtained.
When acrylic resins have a core-shell structure wherein the Tg of a shell-forming polymer is 60xc2x0 C. or higher, it is then possible to use as a core-forming polymer a polymeric material having a Tg value of lower than 60xc2x0 C. such as polyinethyl acrylate (Tg: 3xc2x0 C.), polyethyl acrylate (Tg: xe2x88x9222xc2x0 C.), poly(n-propyle acrylate)(Tg: xe2x88x9244xc2x0 C.) and poly(n-octadecyl methacrylate)(Tg: xe2x88x92100C.). It is here understood that the core-to-shell composition ratio is not critical.
The non-halogen type thermoplastic resin (B) used herein may be easily prepared by emulsion polymerization processes (including a seeding emulsion polymerization process) or fine suspension polymerization processes (including a seeding fine suspension polymerization). To prepare polymer particles suitable for dusting agents and having a small average particle diameter, it has so far been ordinary to make use of a process wherein crude particles or pellets of that polymer are crushed by means of turbo mills, roller mills, ball mills, centrifugal crushers, pulverizers or the like while they are cooled with dry ice, liquid nitrogen or the like to obtain particles, the particles are classified through a classifier, and the thus classified particles are regulated by further pulverization to the desired particle diameter. However, this process is very low in terms of not only productivity but cost-effectiveness as well thanks to the use of dry ice or liquid nitrogen.
Production processes such as emulsion polymerization processes or fine suspension polymerization processes, on the other hand, enable particle diameters to be easily designed depending on the type and amount of the emulsifiers used for polymerization, stirring conditions upon polymerization, etc., so that particles having an average primary particle diameter of 0.1 to 10 xcexcm best suited for the dusting agent of the invention can be easily obtained. In addition, these processes make it possible to prepare particles as close to true spheres as possible. Thus, these production processes are preferable for the preparation of the non-halogen type thermoplastic resin (B) used herein.
If a resin compatible with the olefin resin (A) is used as the non-halogen type thermoplastic resin (B), there is then little or no degradation in the physical properties of the molded product even when used in an increased amount. The aforesaid acrylic resin should preferably have an alkyl group component having eight or more carbon atoms, because its compatibility with the olefin resin increases gradually as the number of carbon atoms in the alkyl group exceeds eight.
In powder slush molding, a powder form of resin composition is fed to a heated mold wherein the composition is melted and deposited onto the surface of the mold. Excessive powders, if any, are fed from the mold back to a reservoir for repeated use in the next mold. In this case, the temperature of the powders fed back to the reservoir rises to about 40 to 60xc2x0 C. under the influence of heat transmitted from the mold. Accordingly, the non-halogen type thermoplastic resin (B) used as the dusting agent should most preferably have a Tg value in the range of 100 to 120xc2x0 C. In this context, an acrylate resin composed mainly of methyl methacrylate and a styrene resin composed mainly of styrene are preferred.
It is herein understood that, in addition to the aforesaid olefin resin (A) and non-halogen type thermoplastic resin (B), thermoplastic elastomers such as urethane-based thermoplastic elastomers and polyester-based thermoplastic elastomers; rubber components such as styrene-butadiene rubber, acrylic rubber, isoprene rubber, butyl rubber and ethylene-propylene rubber, wherein the double bonds of conjugated diene monomers may have been hydrogenated; and plasticized oils based on paraffin, naphthene, aromatics and plants may be used for the purposes of lowering the softening point of the molded product, enhancing the mechanical strength of the same, improving the feel of the same, etc., provided that they should not be detrimental to the objects of the invention. If required, antioxidants, UV absorbers, antitstatics, flame-retardants, pigments, slip agents, dispersants, fillers and other additives may be added to the resin composition. Known plasticizers and so on, too, may be added to the resin composition on condition that they should not be detrimental to moldability and physical properties.
Furthermore in the present invention, organic peroxides may be added to the resin composition for the purposes of lowering the molecular weight of the olefin resin (A) and improving the melting properties of the same.
The olefin resin composition for powder molding of the invention may be produced by uniformly mixing the aforesaid respective components. First, one or two or more olefin resin (A) components are mixed with other components added if required. Any desired mixing means may be used to this end provided that the desired uniform mixture is obtainable. Usually, uniform mixing is carried out by means of mixers such as tumbling mixers or Henschel mixers to obtain a uniform mixture. While the polymer is molten, the mixture is kneaded using a closed type mixture such as Banbury mixers or pressing kneaders or an extrusion kneader such as uniaxial or biaxial extruders, thereby obtaining a powdery mixture free from the non-halogen type thermoplastic resin (B).
For mixing, melting and kneading, it is acceptable to rely upon a process using an extruder having a multiplicity of feed ports, wherein the respective components are successively fed, molten and kneaded.
When an extruder or the like is used with a process for carrying out mixing and kneading while the resin component is molten, fine strands (filaments) of polymer melt are cut by direct use of a fast rotary cutter blade or the like into powders having a diameter of 50 to 500 xcexcm on average. Alternatively, the polymer melt may first be formed into pellets of about 1 to 10 mm in length and about 0.3 to 3 mm in diameter, which are in turn formed into powders having a particle size of 50 to 500 xcexcm on average. For crushers, use may be made of turbo mills, roller mills, ball mills, centrifugal crushers, pulverizers, etc.
Then, the aforesaid powdery mixture is mixed with the non-halogen type thermoplastic resin (B) component. For this mixing, use may be made of mixers unaccompanied by polymer melting such as tumbling mixers, universal mixers, hopper mixers and Henschel mixers.
The thus obtained powdery olefin resin composition should have an average particle diameter in the range of preferably 50 to 500 xcexcm, and more preferably 100 to 300 xcexcm. The xe2x80x9caverage particle diameterxe2x80x9d of the olefin resin composition is here understood to refer to a particle diameter corresponding to an aperture or opening at which a cumulative particle diameter distribution obtained by sieve analysis using a JIS standard sieve indicates 50%. Resin powders of less than 50 xcexcm as represented by this average particle diameter are poor in efficiency of comminution, and are susceptible to agglomeration as well upon prepared and stored. On the other hand, powders of greater than 500 xcexcm tend to yield molded products of coarse texture or give rise to pinholes in molded products when they are thin.